This invention relates to intraocular lens assemblies for implantation into the posterior chamber of a human eye and, more particularly, to accommodating lens assemblies for implantation during cataract eye surgery.
Heretofore, conventional cataract surgery has involved the removal of the normal lens of a human eye and replacement thereof with an artificial intraocular lens assembly which does not have the the ability to change shape for focusing at different distances, as can the normal lens of the eye. The normal lens has the capability of focusing objects in a range of distances which varies from a near point of about 50 millimeters to a far point of infinity. This focusing effect is accomplished by a process known as accommodation. However, the heretofore standard replacement lenses have been fixed focus lenses designed for either distant or close-up vision and have lacked the ability to achieve accommodation. Thus, a patient, having one of these prior lenses implanted during surgery, typically has been required to wear appropriate conventional bifocal or reading glasses thereafter to compensate for the non-accommodating nature of the implant.
U.S. Pat. No. 4,254,509 discloses an intraocular lens structure which is taught to provide properties of accommodation. The lens structure therein includes an optical lens portion which is incapable of changing its curvature since it is formed from a rigid methyl methacrylate-type material such as polymethyl methacrylate ("PMMA"). This rigid, fixed configuration lens portion is supported by haptics which are integrally formed with coplanar oppositely directed feet. The haptics are constructed from a soft, relatively flexible material such as soft hydrogels of hydrophylic type including 2-hydroxyethyl methacrylate ("PHEMA"). These supporting haptics have an archlike configuration, convex side facing the cornea, so that the optic will not touch the iris but will be slightly anterior to it when the structure is implanted in the anterior chamber of the eye. Focusing power of this prior assembly is said to change through anterior movement of the optic resulting from central compressive force exerted on the feet and translated through the soft haptics to the lens upon contraction of the ciliary muscle when the lens is implanted in the anterior chamber of the eye. That is, the patent teaches that central compression of the soft haptics of the lens implanted in the anterior chamber of the eye displaces the lens forward and, thereby, increases the optical power of the system by moving the focusing lens away from the retina. However, it has been found that this system when implanted as taught in the patent does not solve the problem of providing an operationally effective accommodating lens assembly.
A disadvantage of the prior lens assembly is that it is configured for implantation into the anterior chamber of the eye wherein the feet of the haptic supporting the lens are in the anterior chamber angle and, therefore, are effectively in contact with the scleral spur or iris root. As described, the feet of the lens thus positioned in the anterior chamber are pushed centrally during accommodation by an amount sufficient to result in a useful increase in accommodative power. However, it has been found that contraction of the ciliary muscle does not move the scleral spur or iris root centrally or anteriorly. Accordingly, insufficient central movement occurs at the periphery of the anterior chamber to achieve the necessary accommodation with these lens assemblies. Furthermore, to the extend that there is any central displacement of these lens assemblies, less than a millimeter of anterior movement can be achieved in the anterior chamber of the eye when clearance of the iris and cornea are considered. This amount of movement would provide less than about 1 diopter of accommodation utilizing these lens assemblies whereas at least about 3 diopters of accommodation are required to allow focus to go from distance to normal reading vision and about 5 diopters to achieve this degree of accommodation without asthenopia (discomfort resulting from eye strain). Indeed, at least about 8 diopters is preferred to develop an operationally effective accommodating lens system in view of the difficulty in predicting a lens power which would result in ideal distance vision.